Technical Field
The present invention relates to non-destructive testing, and in particular to a method and apparatus for the inspection of electrically conductive components. Applications of the invention include the inspection of tubular components that are often remotely located within the oil and gas exploration and production industries.
State of the Art
A variety of components are employed within the oil and gas industry, such as casings, production tubing, pipelines, flexible risers and steel wire ropes. In order to monitor the structural integrity of these components a variety of non-destructive testing techniques are known for the detection and identification of defects and/or fatigue in the external wall of these tubular components.
One such non-destructive testing technique known in the art is ultrasound inspection, for example as described in U.S. Pat. No. 4,162,635 or US patent publication no. US 2008/0313915. The ultrasound signal is transmitted into the tubing wall, and analysis of the signal reflected from the opposing wall allows information on the wall thickness to be derived. A number of different ultrasonic tools and methods are available, but there are drawbacks and deficiencies associated with their operation. Firstly, ultrasonic tools operating according to the contact method require good coupling between the contact transducers and the test object, and the large mismatch in the acoustic impedance of air and the acoustic impedance of the test material must be overcome. This requires the use of a couplant, for example a liquid or gel-like material that has a low acoustic impedance mismatch and therefore good acoustic coupling between the transducers and the tubular.
Ultrasonic inspection tools are also highly sensitive to dirt and debris, which can interfere with the acoustic coupling and/or show up as anomalous features or artefacts in the analyzed data. This means that ultrasonic inspection may not be practicable for some oil and gas exploration environments.
A second non-destructive testing technique known in the art is magnetic flux leakage testing (MFL). The basic principle is that a powerful magnetic circuit is used to magnetize the component to be tested. At areas where there is corrosion or missing metal, the magnetic field “leaks” from the component and is detected by the MFL probe. The method is therefore limited to use with ferromagnetic materials.
Typically the MFL probe consists of one of two types of magnetic pickups: a coil type or a Hall element. The coil type sensor picks up the rate of change of flux while the Hall type sensor picks up absolute magnetic field. Since the coil output is proportional to the rate of change of flux, the signal is dependent on the scanning speed. At low speeds the coils can totally miss long areas of wall loss if the changes in wall thickness are gradual. The Hall element sensor has no such restrictions.
The output of the MFL sensors is related to change of flux caused by the defect volume, but not directly by defect depth. This technique is therefore an indirect measurement of flaw size. For a proper repeatable signal it is important to magnetize the test component to a very high level (saturation). For pipe types with high wall thickness or thick coating, this is often not possible. The MFL measurement is thus limited to use with certain pipe types.
A third non-destructive testing technique known in the art is eddy current testing (ECT). ECT is based on the principle of measuring the absolute or relative impedance Z of a probe or sensor that comprises a conducting coil to which an alternating current is applied. When the alternating current is applied to the probe a magnetic field develops in and around the coil. This magnetic field expands as the alternating current rises to a maximum and collapses as the current is reduced to zero. If another electrical conductor (the apparatus to be tested) is brought into close proximity to this changing magnetic field, electromagnetic induction takes place and eddy currents (swirling or closed loops of currents that exist in metallic materials) are induced within the apparatus to be tested. The eddy currents flowing in the test material generate their own secondary magnetic fields which oppose the primary magnetic field of the coil and thus change the impedance detected by the probe. This entire process can occur from several hundred times to several million times each second depending on the frequency of the applied alternating current.
In general, the probe is initially balanced on a defect free area of the apparatus to be tested. The probe is then moved relative to the apparatus and variations in the probe impedance Z are recorded. At regions of discontinuities (defects, material property variations, surface characteristics etc,) the flow of the eddy currents is distorted and hence a change of the impedance Z is measured by the probe.
For ECT techniques the probes can be configured in two different operational modes: referred to as absolute and differential modes. Absolute probes generally have a single test coil that is used to generate the eddy currents and sense changes in the eddy current field as the probe moves over the apparatus being tested. Absolute coils are generally suited for measuring slowly varying proprieties of a material. In particular they can be used for conductivity analysis, liftoff measurements, material property changes and thickness measurements.
Differential probes have two active coils usually wound in opposition. When the two coils are over a flaw-free area of test sample, there is no differential signal developed between the coils since they are both inspecting identical material. However, when one coil is over a defect and the other is over good material, a differential signal is produced. Differential probes therefore have the advantage of being very sensitive to localized defects yet relatively insensitive to slowly varying properties such as gradual dimensional or temperature variations.
ECT is an excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known. However, ECT does have some inherent limitations. For example the techniques are only applicable to conductive materials; they require the surface to be tested to be accessible to the probe; and they are limited in the depth of penetration into the material being tested that can be achieved.
Partial Saturation Eddy Current Testing (PSET) is a particular type of eddy current test. PSET techniques employ conventional eddy current coils to monitor the impedance levels within a ferromagnetic material that is being tested. The eddy current coils are however located between two poles of an electromagnet and the electromagnet is arranged to apply a DC magnetic field to the material in the region being monitored by the eddy current coils. The principle behind the PSET technique is that when the ferromagnetic material is magnetised by the DC electromagnet the permeability within the material is changed. When a defect is present the magnetic field generated by the electromagnet experiences a higher flux density, analogous to the situation where a stone is placed in a river causing the water flow to divert around it. This higher flux density causes a change in the localized relative permeability and so distorts the induced eddy current fields in the material which is then detected as a change of the impedance Z measured by the probe.
PSET effectively monitors the relative change in the permeability of a material and so this technique is inherently less sensitive to gradual material property changes. It is therefore particularly effective when operated in a differential mode for the detection of localized discontinuities, such as those caused by cracks, pits and defects.
Since PSET is a relative or comparative technique, the system must be calibrated on reference samples with artificial damage and defects so as to identify the type and severity of defect. However, in practice the material of the reference sample and the test sample may be different. For example, the reference sample may have a relative permeability of 2,500 H m−1. However the inspection pipe may have a relative permeability of 2,000 H m−1. As a result with conventional PSET techniques the identified defect often needs to be determined or corroborated by an alternative NDT technique, for example by ultrasound testing, since the relative permeability of the pipe is usually not known. Often this is not a viable option and even when available it is time consuming and expensive.
Theoretically, PSET can also be operated within an absolute mode. However there is a known inherent problem associated with such tests. When carrying out an absolute mode PSET false hits are known to occur; i.e. a defect can be indicated when one does not truly exist. The reason for these false hits is the fact that PSET readings can be influenced by material property changes. These may include changes in electrical conductivity or changes in the grain structure, for example due to the effects of fatigue within the material. These material property changes affect the relative permeability of the material which in turn is then detected during the absolute mode PSET. The absolute mode PSET cannot however distinguish inherent material property changes from genuine problems such as wall loss. This is because the PSET does not directly measure changes in permeability, it only obtains an apparent change in permeability due the effect this has on the induced eddy currents. Thus, this apparent change could equally well be a result of a material property change or a wall loss, or indeed a combination of the two.
Theoretically, similar false readings can occur during PSET operated in a differential mode if the material property change occurs within a very localized area. However, in reality the frequency of such false readings is much lower than those described in relation to an absolute mode of operation.
The nature of the oil and gas exploration and production industry is such it is expensive and time consuming to remove or replace these components. Therefore, it is highly desirable to be able to carry out any non-destructive testing of the components while they are in situ. Furthermore, in order to obtain the best results it is vital to be able to mount the sensing apparatus as close as possible to the surface of the components to be tested. Over and above the above mentioned limitation of ultrasound testing, MFL testing, ECT and PSET these factors provide further logistical issues when deploying and operating the sensing apparatus, particularly within remote environments. For example, with MFL testing and PSET it is often necessary to locate, maintain and power heavy electromagnets in close vicinity to the components which may be located subsea.
A schematic representation of a flexible riser 1 is provided in FIG. 1. Flexible risers are an example of a component employed to transport hydrocarbons, normally from a well head or manifold on the sea-bed to a floating production platform. These components need to be flexible in order to accommodate the movement of the floating production equipment on the sea surface. They are made of several layers of steel wires that can move with respect to each other. Typically the flexible riser 1 comprises the following layers: an outer thermoplastic sheath 2; first 3a and second 3a longitudinal armament layers in this example separated by a first intermediate thermoplastic sheath 4a; a second intermediate thermoplastic sheath 4b; a radial armament layer 5 (commonly referred to as the zeta layer or zeta wire); an inner thermoplastic sheath 6 and an internal stainless steel carcass 7.
Due to the multi-layer design of the flexible riser 1, it is difficult to inspect all of the components contained therein with the non-destructive testing apparatus known in the art. Operators of flexible risers 1 are particularly concerned with the early detection of defects such as cracks, corrosion, erosion and fatigue within the different layers under various tensional stress levels. Thus, even if the previously described non-destructive inspection apparatus can be deployed with the flexible riser 1 they would only be able to inspect the layers to which they can gain physical access, with the inner layers remaining uninspected.
Furthermore, internal inspection of the flexible riser 1 by MFL testing techniques is not possible because the internal stainless steel carcass 7 comprises an unmagnetizable interlocking layer. The closest magnetizable layer would typically be located several millimeters away from the sensor. From the outside the outer thermoplastic sheath 2, typically made from polyethylene, leads to the same problem and thus flexible risers 1 demands the employment of a different inspection technology. In addition, the structure of the layers consisting of single wires wound in different directions represents a magnetic anisotropy. This means that the so-called zeta wire 5 which is responsible for the strength of the pipe against internal pressure is more difficult to magnetize as compared to a solid steel pipe wall.
One aim and object of aspects of the present invention is to provide a method and apparatus which overcomes or mitigates the drawbacks of prior art non-destructive testing techniques. A further aim and object of aspects of the invention is to provide an alternative method and apparatus to those proposed in the prior art and in particular one that is suited for deployment in situ with components located within remote environments. A further aim and object of aspects of the invention is to provide a non-destructive testing method and apparatus that is suitable for use with a flexible riser. Additional aims and objects will become apparent from reading the following description.